This invention relates to color cathode ray picture tubes and is addressed specifically to an improved method for making front assemblies for color tubes having shadow masks of the tension foil type in association with a substantially flat faceplate. The invention is useful in color tubes of various types, including those used in home entertainment television receivers, and in medium-resolution and high-resolution tubes intended for color monitors.
The use of the foil-type flat tension mask and flat faceplate provides many benefits in comparison to the conventional domed shadow mask and correlatively curved faceplate. Chief among these is a greater power-handling capability which makes possible as much as a three-fold increase in brightness. The conventional curved shadow mask, which is not under tension, tends to "dome" in picture areas of high brightness where the intensity of the electron beam bombardment is greatest. Color impurities result as the mask moves closer to the faceplate and as the beam-passing apertures move out of registration with their associated phosphor elements on the faceplate. The tension mask when heated distorts in a manner quite different from the conventional mask. If the entire mask is heated uniformly, there is no doming and no distortion until tension is completely lost; just before that point, wrinkling may occur in the corners. If only portions of the mask are heated, those portions expand, and the unheated portions contract, resulting in displacements within the plane of the mask; i.e., the mask remains flat.
The tension foil shadow mask is a part of the cathode ray tube front assembly, and is located closely adjacent to the faceplate. The front assembly comprises the faceplate with its screen consisting of deposits of light-emitting phosphors, a shadow mask, and support means for the mask. As used herein, the term "shadow mask" means an apertured metallic foil which may, by way of example, be about 0.001 inch thick, or less. The mask must be supported in high tension a predetermined distance from the inner surface of the cathode ray tube faceplate; this distance is known as the "Q-distance". As is well known in the art, the shadow mask acts as a color-selection electrode, or parallax barrier, which ensures that each of three beams lands only on its assigned phosphor deposits.
The requirements for a support means for a foil shadow mask are stringent. As has been noted, the foil shadow mask is normally mounted under high tension, typically 30 lb/inch. The support means must be of high strength so the mask is held immovable because an inward movement of the mask of as little as 0.0002 inch can cause the loss of guard band. Also, it is desirable that the shadow mask support means be of such configuration and material composition as to be compatible with the means to which it is attached. As an example, if the support means is attached to glass, such as the glass of the inner surface of the faceplate, the support means must have a coefficient of thermal expansion compatible with the glass, and by its composition, be bondable to glass. Also, the support means should be of such composition and structure that the mask can be secured to it by production-worthy techniques such as electrical resistance welding or laser welding. Further, it is essential that the support means provide a suitable surface for mounting and securing the mask. The material of which the surface is composed should be adaptable to machining or other forms of shaping so that it can be contoured into near-perfect flatness so that no voids between the metal of the mask and the support structure can exist to prevent the positive, all-over contact required for proper mask securement.
Means for securing the shadow mask support to the inner surface of the faceplate may comprise a cement in the form of a devitrifying glass solder. (Solder glasses are also commonly known as "frits"). While satisfactory in the main, a cement of this type has significant disadvantages in that it is difficult to handle and apply in production, and it tends to create "pockets" in which screening fluids may lodge and be released later as contaminants. The cathodes of the electron gun are particularly susceptible to poisoning from contaminants. Also, deep-lying ones of such pockets may be connected to the surface of the solder glass by a tiny conduit. The air retained in the pocket may not be depleted during the exhaust cycle, but slowly leak out through the conduit after the tube is sealed, resulting in a reject as a "gassy" tube.
In U.S. Pat. No. 3,894,321 to Moore, of common ownership herewith, there is disclosed means for mounting a foil shadow mask on "rails" which extend from the faceplate. In another embodiment, the faceplate is shown as having an inner ledge that forms a continuous path around the tube, the top surface of which is a Q-distance away from the faceplate for receiving the foil mask.
In U.S. Pat. No. 4,695,761 to Fendley, of common ownership herewith, a foil shadow mask support structure is disclosed in which the structure has a cross-section in the form of an inverted "V", the narrow end of which provides for receiving the mask, and wherein the wide, open end is secured to the faceplate. The means of securement is by fillets of solder glass. Other foil mask support structure embodiments are also disclosed, such as a hollow tube and a rectangle.
In U.S. Ser. No. 942,336, filed Dec. 16, 1986, now U.S. Pat. No. 4,745,328 issued May 17, 1988, of common ownership herewith, there is disclosed a support structure for a tensed foil shadow mask comprising an inverted channel member of metal with a stiffening core of a material such as ceramic secured within, and lateral to, the channel member. In one embodiment of the invention, the space between the stiffening core and the inner walls of the channel is filled with a devitrified glass frit. In another embodiment, a ceramic slurry is poured into a V-shaped support member and is allowed to set, with the object of stiffening the support structure. Except for the area of the structure that contacts the faceplate, the ceramic is totally enclosed within the support structure. A devitrified glass frit may be used as a stiffening material, similarly enclosed within the structure.
In the patent application, U.S. Ser. No. 178,175, filed Apr. 6, 1988, there is disclosed a method for attaching a rectangular rail with a generally V-shape cross-section to a CRT faceplate by partly filling the rail initially with a low viscosity solder glass and after drying overfilling it with a high viscosity solder glass.
The low viscosity initially dispensed solder glass described in this method is suggested as sealing glass No. 7950 manufactured by Corning Glass Works, or alternatively a low viscosity glass designated CV-685 manufactured by Owens-Illinois Television Products. This low viscosity solder glass is sometimes referred to as the "filler frit". The high viscosity solder glass, which overfills the rail when dispensed, is suggested in this application as alternatively being Corning premixed sealing glass No. PM7590 or solder glass CV-695 manufactured by Owens-Illinois. These have viscosities in the range of 500 to 700 poise, while the low viscosity solder glasses are in the range of 160,000 to 240,000 centipoise.
While the method described in this parent application has been found acceptable in production, there have been a margin of tube rejects due in part to voids created in the solder glass in both the filler frit and the high viscosity outer frit.
Microphotographs of cross-sections through the rails and frit indicate voids of 0.010 to 0.020 inches across, and any of these voids near the surface of the seal fillet are subject to bursting when the tube is exhausted in preparation for applying an aluminized coating over the rail seal.
All of the completed CRT tubes are tested for the presence of a residual gas in the faceplate funnel envelope, but these bursting voids are too small to cause a test responsive residual gas reject, and hence, in many cases remain undetected. However, even though residual gas presence remains undetected, the bursting voids cause charged particles in the tube, as well as plugged masks. It has been observed that the solder glass particles coming out of the rail seal at bursting are not as harmful as the burst particles of aluminum coating over the seal that are torn loose upon void burst.
Another problem in the attachment of mask supporting rails to faceplates utilizing this two-layer glass solder system is the formation of air entraining voids formed in the filler frit directly at the periphery of the rail near its apex.
During processing of the tube, small fissures form connecting some of these small voids with the main volume of the tube. Since these small voids are difficult to pump through the restricted fissures, they tend to degas into the finished tube giving objectionably high gas pressures. Attempts at defining a solution to this problem were for a long time unfruitful although slowing the frit dispensing rate produced a nominal reduction in these voids.
It is a primary object of the present invention to minimize the production of voids in the frit filling of rails forming the supporting structure for tension masks in CRTs.